I agree with JockScot. A ceilidh by definition is an informal affair and one would dress (down) accordingly. Any other event would be a dance, which again by definition would be slightly more formal. If there was a meeting or dinner followed by dancing, this would be usually be advertised as a dance, rather than a ceilidh.

But as in all things on Scottish dress, you wear what you wish whilst observing some of the basic protocols.